1. Field of the Invention
The present invention relates to a method of driving a display apparatus having a phosphor layer which is excited by an electron beam generated from a flat electron source and, more particularly, to a display apparatus driving method for a display panel having a phosphor layer excited by an electron beam which is generated due to a field emission of electrons, the method substantially reducing a concentration of electrons on a particular point of the phosphor layer to prevent the phosphor layer from being decreased in the luminous efficacy.
2. Description of the Related Art
As a display panel having a phosphor layer excited by an electron beam, a cathode ray tube, a so-called Braun tube, is available as a well-known apparatus. The Braun tube has a high response speed and wide viewing angle characteristics, and is an emission type display apparatus. For these reasons, this apparatus has been widely used as a high-quality imaging apparatus for a TV set. However, as the screen size of the Braun tube increases, its weight and depth dimension increase. It has therefore been considered that 40-inch size is the limit, and 30-inch size is the limit for home use. On the other hand, the TV system is undergoing a shift from the NTSC system to the high-definition system. With an improvement in the quality of video signals, demands have arisen for low-profile, lightweight, and large-screen display apparatuses.
As a low-profile display apparatus capable of providing high-quality pictures on a large screen, a plasma display panel (PDP) has been commercialized. The PDP can realize a large-screen panel at low cost, because interconnection lines and pixels can be formed by a printing technique. In the PDP, electrical discharges are generated in respective pixels, and ultraviolet rays are generated in the pixels. The ultraviolet rays excite phosphor layers, and light rays are emitted from the phosphor layers to display an image. The PDP displays pictures based on a principle of displaying pictures similar to that for the Braun tube. The PDP, however, is considered to have the following problems. (1) Since a phosphor of the PDP is excited to emit light on the basis of irradiation of ultraviolet light, the luminous efficacy of a phosphor material is low, and the power consumption is high. (2) In the PDP, since the discharge time is very short, in order to obtain a desired luminance, discharge must be repeated for the same pixel. In order to realize a high luminance, emission must be repeated during each field period. A plurality of number of times of this discharge may result in an unnatural movement of a moving picture. (3) In the PDP, the discharge voltage is as high as about 200 V, and hence a high breakdown voltage driver IC is required. As a consequence, the cost of a driver IC tends to be relatively high.
As a large-screen, low-profile display which has currently received attention, a flat display apparatus having a phosphor layer to be excited by an electron beam using a flat electron source is available. The basic structure, manufacturing method, and driving method of this flat display apparatus are disclosed in E. Yamaguchi et al., “A 10-in. SCE-emitter display”, Journal of SID, Vol. 5, p. 345, 1997.1. As reported by E. Yamaguchi et al., the flat display apparatus has the following characteristics. (1) An element array for emitting electrons can be formed by printing. (2) The apparatus uses substantially the same emission principle as that for a Braun tube having a phosphor layer excited by electrons to emit light. (3) In addition, a flat electron source can be driven by a voltage of ten-odd V, and hence allows the use of a low-breakdown-voltage driver IC.
As disclosed by E. Yamaguchi et al., in a phosphor display apparatus using flat electron sources, a matrix of flat electron sources is formed on a glass substrate serving as a rear plate. Each flat electron source is constituted by a pair of element electrodes arranged adjacent to each other and an element film formed between the element electrodes and on the element electrodes. The flat electron source is driven by a voltage applied between the pair of element electrodes to emit electrons from an electron emitting portion formed in the element film. A glass substrate called a faceplate is placed to oppose the rear plate, and the faceplate is coated with phosphor layers, which emit red (R), green (G), and blue (B) light beams for each pixel. Anode electrodes made of aluminum are formed on the phosphor layers. A vacuum is held between the two plates. Electrons emitted from each flat electron source are accelerated by an anode voltage and strike the phosphor layer. The phosphor is excited by the energy of the accelerated electrons to emit light. The emission principle of this flat display apparatus is the same as that of a Braun tube. In the Braun tube, an electron beam emitted from an electron gun is deflected by a deflection coil to scan the screen with the electron beam. In contrast to this, in the phosphor display apparatus using the flat electron sources, electrons are emitted from the flat electron source provided for each pixel, and the phosphor layer corresponding to each pixel is excited to emit light. In addition, the phosphor display apparatus greatly differs from the Braun tube in that the rear and faceplates are held at a distance of about sever mm so as to be a low-profile display apparatus.
As has been described above, this electron source includes a pair of opposing element electrodes, an element film, and an electron emitting portion formed in the element film. A given drive voltage Vf is applied to the pair of element electrodes to emit electrons from the electron emitting portion. A flat display apparatus using such electron sources is characterized in that a voltage that starts electron emission is as low as about 10 V, and a voltage that is used to obtain an electron emission amount required for the phosphor to emit light with a sufficient luminance is as low as ten-odd V. In the flat display apparatus, an emitted electron is influenced by a force acting from the low-potential side of an element electrode to the high-potential side, and the emitted electron is displaced and travels to the anode electrode. As a consequence, the electron forms a curved locus having a given directionality. This produces a deviation between the irradiation position of the electron on the faceplate and the position of the electron emitting portion of the electron source.
A display apparatus having a phosphor layer excited by an electron beam emitted from such a flat electron source uses phosphor excitation/emission by an electron beam with high luminous efficacy, and hence consumes only a small amount of power even with a large screen. In addition, when a phosphor emits light, a raster emits light for a selected very short period of time. Since this display is not of a hold type as in a liquid crystal display apparatus (LCD) and PDP, natural pictures can be displayed even in moving picture display operation. In addition, the screen luminance of this apparatus has no viewing angle dependence as in an LCD, and hence the apparatus has wide viewing angle characteristics. Furthermore, since a flat electron source can be operated at ten-odd V, it can be driven by a low-voltage driver IC.
As described above, electrons emitted from the electron emitting portion of each electron source are injected into the anode electrode. When such an electron is emitted, a directionality is given to the electron such that it is attracted to one of the pair of element electrodes which is on the high-potential side. The emitted electron therefore has not only an initial velocity component directed to the anode electrode but also an initial velocity component displaced toward the electrode on the high-potential side. As a consequence, the emitted electron forms a curved locus and travels toward the anode electrode to reach the anode electrode at a position displaced from a position on the anode electrode which is immediately above the electron emitting portion and opposes it.
The actual emission pattern generated by this emitted electron has an emission peak at a position deviated from the geometric center of the pattern, and has a distribution in which the luminance is monotonously attenuated from the emission peak as the center. For this reason, at a position where an emission peak appears, the anode current density is always high. Even with the same operation time, therefore, a large quantity of electrons are injected into a portion of the phosphor layer which corresponds to this position. It is generally known that the emission luminance of a phosphor decreases in accordance with the amount of electric charge injected. For this reason, at a position where the anode current density is high, the luminous efficacy abruptly decreases, resulting in a decrease in the luminance of pixels. Although a region where this emission peak appears is small in area, the region corresponds to a region in which a large amount of electric charge is injected. In addition, the ratio of this region which contributes to overall emission luminance is higher than the area of the region which contributes to the overall emission luminance. For this reason, a further decrease in luminance occurs in accordance with the emission intensity, and the overall luminance decreases quickly.